Question 1

For clock puzzles, how many times in one day are the hands of a clock in a straight line but pointing in opposite directions?

Accepted Answer

Introduction / Context: This puzzle question belongs to the standard topic of clock problems in quantitative aptitude. It asks how many times in a full day the hour hand and the minute hand of a clock lie in a straight line but are pointing in opposite directions. Understanding the relative motion of clock hands and the patterns they follow over twelve hours and twenty four hours is essential for solving many related questions. Given Data / Assumptions: The clock is a regular twelve hour analog clock with an hour hand and a minute hand. The phrase in a straight line but opposite in direction means the two hands are separated by exactly 180 degrees. The time period of interest is one full day of twenty four hours. We assume the clock runs uniformly with no mechanical errors. Concept / Approach: The basic concept is that the minute hand and hour hand move at different speeds. The minute hand moves 360 degrees in 60 minutes, that is 6 degrees per minute, while the hour hand moves 360 degrees in 12 hours, that is 0.5 degrees per minute. The relative speed between them is 5.5 degrees per minute. In clock problems, alignments such as hands together or hands in opposite directions occur at regular intervals based on this relative speed. Over a twelve hour period, the hands are in opposite directions 11 times, so over a full twenty four hour day, that count doubles. Step-by-Step Solution: Step 1: Note that the minute hand speed is 6 degrees per minute and the hour hand speed is 0.5 degrees per minute. Step 2: Compute the relative angular speed as 6 minus 0.5 which equals 5.5 degrees per minute. Step 3: For the hands to be in opposite directions, the angular separation between them must be 180 degrees. Step 4: The time between consecutive instances when the hands are opposite is given by 180 divided by 5.5 minutes. Step 5: The total time in twelve hours is 12 hours multiplied by 60 which equals 720 minutes. Step 6: The number of times the hands are opposite in twelve hours is 720 divided by the interval in minutes, which results in 11 occurrences. Step 7: In a full day of twenty four hours, the pattern repeats twice, so the total number of opposite alignments is 11 multiplied by 2 which equals 22. Verification / Alternative check: An alternative way to check is to remember the standard results for clock problems. In twelve hours, the hands coincide 11 times and are opposite 11 times. Therefore, in twenty four hours, the number of coincidences is 22 and the number of times they are opposite is also 22. This rule is widely used in aptitude books and confirms that 22 is the correct count without needing to calculate each time specifically. Why Other Options Are Wrong: 20: This underestimates the actual count and does not follow from the twelve hour pattern of 11 occurrences. 24: This number mistakenly assumes one opposite alignment per hour, which is not accurate because the intervals are slightly more than one hour apart. 48: This value doubles an incorrect assumption of 24 per day and is much higher than the correct count. Common Pitfalls: A common error is to assume that the hands are opposite exactly every hour, leading to 24 as an answer. Another mistake is mixing up the count for hands in the same direction with the count for hands in opposite directions. To avoid these pitfalls, it helps to memorize that in twelve hours, there are 11 coincidences and 11 opposite alignments, giving 22 each over a full day. Using the relative speed method reinforces this pattern and prevents confusion. Final Answer: In one full day, the hands of a clock are in a straight line but opposite in direction 22 times.

Question 2

A clock is started at noon; by 10 minutes past 5, through how many degrees has the hour hand turned from its starting position?

Accepted Answer

Introduction / Context: This question is a classic example of a clock angle problem from quantitative aptitude. It asks how many degrees the hour hand has rotated from noon to ten minutes past five. Understanding the angular speed of the hour hand and how long it has been moving allows you to find the exact angle. Such questions are common in competitive exams that test numerical reasoning and time and work with clocks. Given Data / Assumptions: The clock is started at noon, which is 12:00. The time in question is 5:10, that is five hours and ten minutes after noon. Only the rotation of the hour hand from its starting position at 12:00 is considered. We assume the clock is ideal, and both hands move uniformly at their standard rates. Concept / Approach: The key concept is the speed of the hour hand. A full circle is 360 degrees, and the hour hand completes one full circle in twelve hours. Therefore, the hour hand moves 360 divided by 12 which equals 30 degrees per hour. Equivalently, since each hour is 60 minutes, the hour hand moves 0.5 degrees per minute. By calculating how many minutes have passed between noon and 5:10, you can multiply by 0.5 degrees per minute to find the total angle turned. Step-by-Step Solution: Step 1: Determine the time interval from noon to 5:10. From 12:00 to 5:00 is 5 hours, and from 5:00 to 5:10 is an additional 10 minutes. Step 2: Convert the total time to minutes. Five hours is 5 multiplied by 60 which equals 300 minutes. Add 10 minutes to get 310 minutes in total. Step 3: Recall that the hour hand moves 0.5 degrees each minute. Step 4: Multiply the minutes by the angular speed: angle turned equals 310 multiplied by 0.5 degrees per minute. Step 5: Compute 310 multiplied by 0.5 which equals 155 degrees. Step 6: Compare this result with the answer choices 145, 150, 155, and 160 and identify 155 as the matching value. Verification / Alternative check: Another way to check is to break the motion into hours and minutes separately. For five full hours, the hour hand moves 5 multiplied by 30 which equals 150 degrees. In ten additional minutes, it moves 10 multiplied by 0.5 which equals 5 degrees. Adding these gives 150 plus 5 which equals 155 degrees. This alternative calculation confirms the earlier result and ensures that there is no arithmetic mistake. Why Other Options Are Wrong: 145: This value could come from miscalculating the hours or minutes, but it does not match the correct total of 155 degrees. 150: This corresponds only to the movement in a full five hours and ignores the extra ten minutes. 160: This overestimates the angle, perhaps by incorrectly using the minute hand speed instead of the hour hand speed. Common Pitfalls: A common mistake is to forget that the hour hand moves continuously and not just in jumps each hour. Students sometimes calculate only the angle for whole hours and ignore the additional minutes, leading to 150 degrees instead of 155 degrees. Another error is mixing up the angular speeds of the minute and hour hands. Memorizing that the hour hand moves 0.5 degrees per minute and 30 degrees per hour can prevent these mistakes. Final Answer: By 10 minutes past five, the hour hand has turned through 155 degrees from its position at noon.

Question 3

A watch that gains uniformly is 2 minutes slow at Monday noon and 4 minutes 48 seconds fast at 2 p.m. the following Monday; at what time was the watch exactly correct?

Accepted Answer

Introduction / Context: This is a classic clock gain and loss problem from quantitative aptitude. The watch in question gains time uniformly, meaning it runs consistently faster than real time. At one reading, the watch is slow; at a later reading, it is fast. The question asks you to determine the exact time when the watch showed the correct time. Such problems test your understanding of uniform rate of gain or loss and how to convert time differences into rate based calculations. Given Data / Assumptions: At Monday noon, the watch is 2 minutes slow compared to the correct time. At 2 p.m. on the following Monday, the same watch is 4 minutes 48 seconds fast. The watch gains time uniformly between these two observations. We assume no resets or interruptions occurred during this period. Concept / Approach: The core idea is to treat the difference between the two states of the watch as the total gain over a known time interval. At the first observation, the watch is 2 minutes behind. At the second observation, it is 4 minutes 48 seconds ahead. The total gain over this interval is therefore 2 minutes plus 4 minutes 48 seconds. Once we know the total gain and the length of the interval, we can compute the rate of gain per hour. Finally, we use that rate to determine how long it takes the watch to gain 2 minutes, which is the amount needed to move from being 2 minutes slow to being exactly correct. Step-by-Step Solution: Step 1: Compute the time interval from Monday noon to 2 p.m. the following Monday. From Monday noon to the next Monday noon is 7 days, or 7 multiplied by 24 equals 168 hours. Adding 2 extra hours gives 170 hours in total. Step 2: Determine the total gain of the watch over this interval. Initially, it is 2 minutes slow. Finally, it is 4 minutes 48 seconds fast. The total gain equals 2 minutes plus 4 minutes 48 seconds. Step 3: Convert 4 minutes 48 seconds to minutes. 48 seconds is 48 divided by 60 which equals 0.8 minutes. So 4 minutes 48 seconds equals 4.8 minutes. Step 4: Add the two parts of the gain: 2 minutes plus 4.8 minutes equals 6.8 minutes of total gain over 170 hours. Step 5: Compute the rate of gain per hour. Rate equals 6.8 divided by 170 minutes per hour, which simplifies to 0.04 minutes per hour, that is 2.4 seconds per hour. Step 6: To find when the watch was correct, calculate how long it takes to gain 2 minutes, because starting from 2 minutes slow, gaining 2 minutes makes it exactly correct. Step 7: Time to gain 2 minutes equals 2 divided by 0.04 hours, which is 50 hours. Step 8: Add 50 hours to the starting point, Monday noon. Forty eight hours from Monday noon takes you to Wednesday noon. Adding 2 more hours gives Wednesday 2 p.m. Step 9: Therefore, the watch was exactly correct at 2 p.m. on Wednesday. Verification / Alternative check: As a check, note that from Monday noon to Wednesday 2 p.m. is exactly 50 hours. At this point, the watch has gained 2 minutes and moved from being 2 minutes slow to exactly correct. From Wednesday 2 p.m. to the following Monday 2 p.m. is the remaining 120 hours. At the gain rate of 0.04 minutes per hour, over 120 hours the watch gains 120 multiplied by 0.04 equals 4.8 minutes. Thus, by the next Monday at 2 p.m. it is 4.8 minutes fast, matching the condition given in the problem. This confirms that our calculation and answer are consistent. Why Other Options Are Wrong: 2 p.m. on Tuesday: This corresponds to only 26 hours after Monday noon. At the gain rate, the watch would not yet have gained the full 2 minutes needed to become correct. 3 p.m. on Thursday: This is much later than the correct time and would yield a gain larger than 2 minutes, so the watch would be fast by then. 1 p.m. on Friday: This time is also far beyond the moment when the watch becomes correct and does not match the uniform gain pattern needed to satisfy both given conditions. Common Pitfalls: One common mistake is to forget to add the initial delay and final advance when calculating total gain, leading to using only 4 minutes 48 seconds instead of 2 plus 4 minutes 48 seconds. Another pitfall is mishandling the conversion of seconds into decimal minutes, which can distort the rate calculation. Finally, some students miscount hours between the two observations or forget that 7 full days are involved. Carefully tracking the time interval, converting all times to minutes, and methodically computing the gain per hour avoids these errors. Final Answer: The watch was exactly correct at 2 p.m. on Wednesday.

Question 4

At 3:40 on an analog clock, what is the angle between the hour hand and the minute hand?

Accepted Answer

Introduction / Context: Clock angle questions are a classic part of quantitative aptitude and reasoning tests. They check whether you understand how fast the hour hand and minute hand move and how to translate a given time into angles on the dial. In this question, we are asked to find the angle between the hour hand and the minute hand when the clock shows 3:40, which means we must carefully account for the fact that the hour hand has moved away from the exact 3 o clock position. Given Data / Assumptions: - Time shown on the clock is 3:40. - A full circle on the clock is 360 degrees. - The minute hand completes one full revolution in 60 minutes. - The hour hand completes one full revolution in 12 hours. - We are interested in the smaller angle between the two hands. Concept / Approach: The minute hand moves 360 degrees in 60 minutes, so it moves 6 degrees per minute. The hour hand moves 360 degrees in 12 hours, which is 360 degrees in 720 minutes, so it moves 0.5 degrees per minute. To find the angle of each hand from the 12 o clock position, we multiply the time in minutes by the appropriate rate and then take the difference between the two positions. The absolute difference gives the angle between the hands, and if needed we choose the smaller angle inside the clock face. Step-by-Step Solution: Step 1: At 3:40, the minute hand is at 40 minutes. Step 2: Angle of the minute hand from 12 is 40 * 6 = 240 degrees. Step 3: At 3:40, the hour hand has moved 3 full hours plus 40 minutes. Step 4: In 3 hours, the hour hand moves 3 * 30 = 90 degrees, because each hour mark is 30 degrees apart. Step 5: In 40 minutes, the hour hand moves 40 * 0.5 = 20 degrees further. Step 6: Total angle of the hour hand from 12 is 90 + 20 = 110 degrees. Step 7: The difference between the hands is |240 - 110| = 130 degrees. Step 8: The reflex angle would be 360 - 130 = 230 degrees, but the standard convention is to report the smaller angle, which is 130 degrees. Verification / Alternative check: Another way is to remember that at exactly 3:00 the hour hand is at 90 degrees. In 40 minutes it moves an extra 40 * 0.5 = 20 degrees, reaching 110 degrees. The minute hand at 40 minutes is at 240 degrees. The same subtraction 240 minus 110 gives 130 degrees. Since 130 degrees is less than 180 degrees, it is clearly the smaller angle between the hands, confirming our result. Why Other Options Are Wrong: 120 degrees is too small because it assumes the hour hand did not move enough from the 3 o clock position. 125 degrees and 135 degrees are near the correct result but do not match the precise calculation using 6 degrees per minute and 0.5 degrees per minute. 150 degrees is even larger and ignores the exact motion of the hour hand during the 40 minutes after 3:00. Common Pitfalls: Many learners forget that the hour hand moves continuously and not in jumps of one hour. They sometimes keep the hour hand fixed at 90 degrees for 3:00 and only move the minute hand, which leads to an incorrect angle. Another common mistake is to compute the reflex angle instead of the smaller interior angle without checking which one the question is asking. Always compute both the absolute difference and, if needed, 360 minus that difference, and then choose the smaller angle unless the question specifies reflex angle. Final Answer: The correct smaller angle between the hour hand and the minute hand at 3:40 is 130 degrees.

Question 5

If today is the reference day, what day is two days after the day after the day before yesterday?

Accepted Answer

Introduction / Context: This puzzle tests your ability to work with relative descriptions of days rather than with named days like Monday or Tuesday. Many reasoning and aptitude exams include such questions to see whether you can carefully track a sequence of time references starting from today. The key is to move step by step, converting each phrase into a simple shift forward or backward in days, and then simplifying the chain into a direct relationship with today. Given Data / Assumptions: - Let today be the reference point, denoted as T. - The phrase day before yesterday represents T minus 2 days. - The phrase day after represents one day ahead of the mentioned day. - We must interpret two days after the day after the day before yesterday in terms of T. - No specific weekday name is given, so the answer should be expressed relative to today. Concept / Approach: The approach is to translate the verbal description into a sequence of simple arithmetic moves on the timeline. Each phrase like before or after means subtracting or adding one or more days from the base day. By using symbols such as T for today and carefully applying each step, we find which relative day the final expression represents. Keeping the calculation purely symbolic avoids confusion about specific weekday names and focuses on logical structure. Step-by-Step Solution: Step 1: Represent today as T. Step 2: The day before yesterday is T - 2 days. Step 3: The day after the day before yesterday is one day after T - 2, which is (T - 2) + 1 = T - 1. This is yesterday. Step 4: Now we need two days after that day. Two days after T - 1 is (T - 1) + 2 = T + 1. Step 5: T + 1 is one day after today, which is tomorrow. Step 6: Therefore, the final expression simplifies to tomorrow. Verification / Alternative check: You can also reason in words. Starting from day before yesterday, move one day forward to reach yesterday. From yesterday, moving two days forward takes you first to today and then to tomorrow. Thus the final day is tomorrow. Another verification method is to pick an example, such as assuming today is Wednesday, then day before yesterday is Monday, day after that is Tuesday, and two days after Tuesday is Thursday, which is indeed tomorrow relative to Wednesday in that example structure. The relative pattern remains the same. Why Other Options Are Wrong: Yesterday corresponds to T - 1, not T + 1. Today corresponds to T, which is not two steps ahead of yesterday. The day after tomorrow is T + 2, and two days ago is T - 2. None of these match T + 1, which is tomorrow. Only the option labelled Tomorrow matches the step by step simplification of the given description. Common Pitfalls: A common mistake is to rush through the wording and mentally skip one of the after or before steps, which leads to answers like today or the day after tomorrow. Another pitfall is mixing up direction, adding when you should subtract or the other way around. It helps to write each step on paper, starting from T and then carefully applying each phrase. Treat such puzzles like small algebra problems to avoid misinterpretation of the language. Final Answer: Two days after the day after the day before yesterday is tomorrow relative to today.

Question 6

How many minutes are left until 6:00 if, fifty minutes ago, the time was four times as many minutes past 3:00 as the number of minutes now remaining until 6:00?

Accepted Answer

Introduction / Context: This is a classic clock and algebra puzzle that links a past time with the number of minutes remaining until a future time. The question connects the unknown number of minutes remaining until 6:00 with how many minutes past 3:00 it was fifty minutes ago. Such puzzles are common in aptitude tests because they test both your understanding of time intervals and your ability to set up and solve a simple linear equation using those intervals. Given Data / Assumptions: - Let x be the number of minutes from now until 6:00. - Fifty minutes ago, the time was (current time) minus 50 minutes. - Six o clock corresponds to 6:00, and three o clock corresponds to 3:00. - At that earlier time, the minutes past 3:00 are given to be four times x. - We assume a normal 12 hour clock with no irregularities. Concept / Approach: The key idea is to express all times in minutes counted from a fixed reference, here from 3:00. By letting x be the unknown minutes until 6:00, we can compute the actual clock time now, then compute the clock time fifty minutes before now, and finally express how many minutes that earlier time was past 3:00. The condition in the question translates into one linear equation in x, which we solve using basic algebra. Once x is found, we compare it to the answer choices. Step-by-Step Solution: Step 1: Let x be the number of minutes from now until 6:00. Step 2: The current time is 6:00 minus x minutes. Step 3: Fifty minutes ago, the time was 6:00 minus x minus 50 minutes. Step 4: Count minutes from 3:00. In minutes, 6:00 is 3 hours after 3:00, that is 180 minutes after 3:00. Step 5: The time fifty minutes ago is therefore 180 minus x minus 50 = 130 - x minutes after 3:00. Step 6: The condition says that at that moment, the minutes past 3:00 equal four times the minutes now remaining until 6:00, so 130 - x = 4x. Step 7: Solve the equation 130 - x = 4x which gives 130 = 5x and hence x = 26 minutes. Step 8: Therefore, there are 26 minutes left until 6:00. Verification / Alternative check: Compute the actual times. If x = 26, the current time is 5:34. Fifty minutes earlier, the time was 4:44. From 3:00 to 4:44 is 1 hour 44 minutes which is 60 + 44 = 104 minutes. Four times 26 is 104, which matches the statement that fifty minutes ago it was four times as many minutes past 3:00 as it is now to 6:00. This confirms that the algebraic solution is consistent with the actual clock times and therefore correct. Why Other Options Are Wrong: If we test 18, 20, 22, or 24 minutes in place of x, the computed time fifty minutes earlier does not give minutes past 3:00 equal to four times the assumed x. For example, with x = 24, the current time is 5:36 and fifty minutes earlier it is 4:46, which is 106 minutes after 3:00, not equal to 4 * 24 = 96. Similar mismatches occur for 18, 20 and 22, so they do not satisfy the condition. Common Pitfalls: A frequent error is to mix up the direction of time shifts, for example adding 50 instead of subtracting it for the earlier time, or forgetting to convert all times to a consistent reference like minutes after 3:00. Another pitfall is misinterpreting the statement and accidentally making x four times the past minutes instead of the other way around. Writing the equation carefully in symbolic form before solving helps avoid these traps. Final Answer: The number of minutes remaining until 6:00 is 26 minutes.

Question 7

Between 9:00 and 10:00, at what exact time will the hands of a clock be 180 degrees apart?

Accepted Answer

Introduction / Context: This question is a typical clock angle problem where we must find the exact time, as a fractional number of minutes past an hour, when the angle between the hour and minute hands is 180 degrees. Such questions test your ability to translate clock positions into degrees and to use a general formula connecting the time after the hour to the angle between the clock hands. Here the time is between 9:00 and 10:00 and we are specifically looking for when the hands are in a straight line but opposite each other, forming an angle of 180 degrees. Given Data / Assumptions: - The time is between 9:00 and 10:00. - We want the moment when the angle between hour and minute hands is 180 degrees. - The minute hand moves 6 degrees per minute. - The hour hand moves 0.5 degrees per minute. - At exactly 9:00 the hour hand is at 270 degrees from the 12 o clock position. Concept / Approach: Let m be the number of minutes past 9:00. We compute the angle of the minute hand and the angle of the hour hand from the top of the clock (12 o clock position) and set their absolute difference equal to 180 degrees. The minute hand angle is given by 6 * m, and the hour hand angle at 9:m is 270 + 0.5 * m. Taking the difference and solving for m gives a linear equation. We then check which of the options matches the value of m derived from the equation. Step-by-Step Solution: Step 1: Let m be the minutes past 9:00. Step 2: Angle of the minute hand from 12 is 6 * m degrees. Step 3: At 9:00, the hour hand is at 9 * 30 = 270 degrees. Step 4: After m minutes, the hour hand moves an extra 0.5 * m degrees, so its angle is 270 + 0.5 * m degrees. Step 5: The angle between the hands is |270 + 0.5 * m - 6 * m| = |270 - 5.5 * m|. Step 6: Set this equal to 180 degrees: |270 - 5.5 * m| = 180. Step 7: Consider 270 - 5.5 * m = 180, which simplifies to 5.5 * m = 90 and m = 90 / 5.5 = 180 / 11 minutes. Step 8: The alternative equation 5.5 * m - 270 = 180 would give m greater than 60 minutes, which is outside the hour from 9:00 to 10:00, so it is rejected. Verification / Alternative check: Compute the actual angles using m = 180 over 11 minutes, which is approximately 16.36 minutes. The minute hand is then at 6 * 180 / 11 = 1080 / 11 degrees, and the hour hand is at 270 + 0.5 * 180 / 11 = 270 + 90 / 11 degrees. The difference between these two angles is exactly 180 degrees when simplified. Since m is within the 0 to 60 minute range after 9:00, this time is valid and satisfies the condition of being between 9:00 and 10:00. Why Other Options Are Wrong: The options involving times past 10:00 cannot be correct because the problem specifically asks for a time between 9:00 and 10:00. The other suggested fractional minutes past 9:00 do not satisfy the equation |270 - 5.5 * m| = 180 when checked. Only m equal to 180 over 11 minutes produces an exact 180 degree angle between the hands within the correct one hour interval. Common Pitfalls: Learners often forget that the hour hand is also moving during the minutes after 9:00 and treat it as fixed at 270 degrees, which leads to an incorrect equation. Another pitfall is failing to consider that the absolute value equation can in principle give two solutions, one of which may be outside the 0 to 60 minute range. Always set up the general formula, solve the resulting equation, and then check whether the solution falls in the specified time window. Final Answer: The hands of the clock will be 180 degrees apart at 180 over 11 minutes past 9:00.

Question 8

When an analog clock shows 4:20 p.m., what is the angle between the hour hand and the minute hand?

Accepted Answer

Introduction / Context: Clock angle problems require you to convert a given time into positions of the hour and minute hands measured in degrees. The problem here asks for the angle between the two hands at 4:20 p.m., which is a time where the hour hand is between the 4 and 5 marks on the dial. Understanding the rates at which each hand moves allows us to compute the exact angle and distinguish between closely spaced options like 5, 10, 15, and 25 degrees. Given Data / Assumptions: - The time on the clock is 4:20 p.m. - The minute hand completes a full circle of 360 degrees in 60 minutes. - The hour hand completes a full circle of 360 degrees in 12 hours. - We want the smaller angle between the hour and minute hands. - The clock is assumed to be accurate and circular with 12 equal hour divisions. Concept / Approach: The minute hand moves 6 degrees per minute because 360 divided by 60 equals 6. The hour hand moves 0.5 degrees per minute because it moves 360 degrees in 12 hours, which is 720 minutes, giving 360 divided by 720 equals 0.5 degrees per minute. To find the angles of both hands from the 12 o clock position, we use these rates and then subtract the smaller from the larger to find the angle between them. If that angle exceeds 180 degrees, we subtract it from 360 degrees to get the smaller interior angle, but in this case that will not be necessary. Step-by-Step Solution: Step 1: At 4:20, the minute hand is at 20 minutes. Step 2: Angle of the minute hand is 20 * 6 = 120 degrees from 12. Step 3: At 4:00, the hour hand is at 4 * 30 = 120 degrees. Step 4: In the 20 minutes after 4:00, the hour hand moves extra degrees equal to 20 * 0.5 = 10 degrees. Step 5: Therefore, the hour hand angle from 12 at 4:20 is 120 + 10 = 130 degrees. Step 6: The difference between the positions of the minute and hour hands is |130 - 120| = 10 degrees. Step 7: Since 10 degrees is less than 180 degrees, it is already the smaller angle between the hands. Verification / Alternative check: We can visualize the clock: the minute hand points at the 4 on the dial (representing 20 minutes) which is at 120 degrees, while in twenty minutes the hour hand has moved one third of the way between the 4 and 5 marks. Each hour division is 30 degrees, so one third of that is 10 degrees beyond the 4. The hour hand is thus slightly ahead of the minute hand. The visual picture agrees with our calculation that the difference in their positions is 10 degrees. Why Other Options Are Wrong: An angle of 5 degrees would require the hour hand to be much closer to the minute hand, which does not match the one third of an hour movement. Fifteen degrees or twenty five degrees would overstate the movement of the hour hand away from the minute hand. The calculation based on exact rates yields 10 degrees, so the other numerical values do not satisfy the actual geometry of the clock at 4:20. Common Pitfalls: A common mistake is to forget the continuous movement of the hour hand and simply keep it fixed at the 4 mark, which would give 120 degrees and lead people to incorrectly think that the hands coincide. Another pitfall is miscalculating the hour hand movement as 20 degrees instead of 10 degrees by confusing the rates. Always remember that the hour hand moves only 0.5 degrees per minute, which is much slower than the minute hand, and use the correct rates in your calculations. Final Answer: At 4:20 p.m., the angle between the hour and minute hands of the clock is 10 degrees.

Question 9

Approximately how many years are there in one billion (1,000,000,000) seconds, assuming a year has 365 days?

Accepted Answer

Introduction / Context: This puzzle invites you to connect a very large measure of time in seconds with an everyday measure in years. It develops a sense of scale and your ability to handle unit conversions between seconds, minutes, hours, days, and years. Estimating how long one billion seconds lasts is also a useful mental exercise for understanding large quantities that often appear in science, computing, and population statistics. Given Data / Assumptions: - Total time to convert is 1,000,000,000 seconds. - One minute has 60 seconds. - One hour has 60 minutes. - One day has 24 hours. - One year is assumed to have 365 days, ignoring leap years for simplicity. Concept / Approach: The standard unit conversion approach is to divide step by step. First convert seconds to minutes by dividing by 60. Then convert minutes to hours by again dividing by 60. Next convert hours to days by dividing by 24. Finally, convert days to years by dividing by 365. At each stage, we keep track of the approximate value. The options are close to one another, so accuracy at each step matters. Using an approximate value for seconds per year gives a compact and efficient method. Step-by-Step Solution: Step 1: Calculate the number of seconds in one year with 365 days. Step 2: Seconds in a day are 24 * 60 * 60 = 86,400 seconds. Step 3: Seconds in a 365 day year are 365 * 86,400 = 31,536,000 seconds. Step 4: To find how many years are in one billion seconds, divide 1,000,000,000 by 31,536,000. Step 5: 1,000,000,000 / 31,536,000 is approximately 31.7 years. Step 6: Among the choices, 31.68 years is the closest to 31.7 years. Step 7: Therefore, the best approximation from the options is 31.68 years. Verification / Alternative check: We can quickly check reasonableness by using a rounded value for seconds in a year, such as 31,500,000. Then 1,000,000,000 / 31,500,000 is about 31.75 years. This once again suggests a value around 31.7 years, consistent with the more precise calculation. When compared to the options, 31.68 years is clearly the best fit, and none of the higher values like 33.31 years or 34.76 years are close enough to the exact quotient to be plausible. Why Other Options Are Wrong: Values such as 34.76 years, 33.31 years, and 32.48 years are all significantly larger than 31.7 years and would correspond to fewer seconds per year than our calculation, which conflicts with the standard conversion from days to seconds. A value lower than 31.68 years would also not match the accurate division of one billion by 31,536,000. Therefore, 31.68 years remains the only option that aligns closely with the computed ratio. Common Pitfalls: Some people mistakenly divide by 365 too early or forget one of the 60 factors when converting seconds to minutes or hours. Others mix up whether to multiply or divide during the conversion chain. It is important to remember that to move from a smaller unit to a larger unit, such as seconds to years, you divide step by step by 60, 60, 24, and 365. Keeping track of units at each stage helps prevent errors and makes the final answer far more reliable. Final Answer: One billion seconds is approximately equal to 31.68 years when using a 365 day year.

Question 10

The minute hand of a clock overtakes the hour hand every 65 minutes. In how many days will the minute hand gain a total of 1440 hours over the hour hand?

Accepted Answer

Introduction / Context: This puzzle is about the relative motion of the hour and minute hands on a clock. It states that the minute hand overlaps the hour hand every 65 minutes and asks in how many days the minute hand will gain a total of 1440 hours over the hour hand. The problem requires understanding that each overlap represents the minute hand gaining one full hour mark over the hour hand, then scaling that process up to a very large total gain measured in hours and finally converting that interval into days. Given Data / Assumptions: - The minute hand overlaps the hour hand every 65 minutes. - Each overlap corresponds to the minute hand gaining one hour mark on the dial relative to the hour hand. - We are asked when the total gain will reach 1440 hours. - One day has 24 hours. - We consider uniform motion of clock hands with no mechanical errors. Concept / Approach: Between two consecutive overlaps, the minute hand gains exactly one full revolution in terms of hour marks relative to the hour hand, which is equivalent to a gain of one hour on the 12 hour dial. Because the overlaps occur regularly approximately every 65 minutes, the total time needed to gain a large number of hours is the number of overlaps multiplied by 65 minutes. Once the total minutes are found, we convert them into hours and then into days in order to match the units requested in the question. Finally we compare with the options provided. Step-by-Step Solution: Step 1: Each overlap of the minute hand over the hour hand corresponds to a gain of 1 hour on the dial. Step 2: The total gain required is 1440 hours. Step 3: Therefore, the minute hand must overlap the hour hand 1440 times to gain 1440 hours. Step 4: The time between overlaps is 65 minutes, so the total time in minutes is 1440 * 65. Step 5: Compute the total minutes: 1440 * 65 = 93,600 minutes. Step 6: Convert minutes to hours by dividing by 60: 93,600 / 60 = 1560 hours. Step 7: Convert hours to days by dividing by 24: 1560 / 24 = 65 days. Step 8: Thus, it will take 65 days for the minute hand to gain 1440 hours over the hour hand. Verification / Alternative check: We can check the logic by considering smaller numbers. If the minute hand gains 1 hour every 65 minutes, then in 24 overlaps it gains 24 hours. The time for 24 overlaps is 24 * 65 = 1560 minutes, which is 26 hours, or a little more than one day. Scaling this up, 1440 overlaps is 60 times 24 overlaps, so the total time becomes 60 * 26 hours, which is 1560 hours. Dividing by 24 hours per day again gives 65 days. The alternative reasoning matches the direct calculation, confirming that 65 days is consistent. Why Other Options Are Wrong: Options such as 57, 58, 60, or 61 days would correspond to fewer total hours than 1560 when multiplied by 24. That in turn would imply fewer than 1440 overlaps at 65 minutes per overlap or a gain smaller than 1440 hours. Since the arithmetic shows that 1440 overlaps require exactly 65 days, the other values do not satisfy the gain condition stated in the problem. Common Pitfalls: One common source of confusion is mixing up hours and minutes in the phrase gain 1440 hours and mistakenly treating 1440 as minutes, which is the number of minutes in a single day. Another pitfall is forgetting that each overlap corresponds to a gain of one hour on the dial rather than one minute. Carefully distinguishing between the gain per overlap and the time between overlaps ensures that the scaling to 1440 hours is handled correctly and that the correct number of days is obtained. Final Answer: The minute hand will gain a total of 1440 hours over the hour hand in 65 days.

Question 11

Between 2:00 and 3:00, at what exact time do the hour hand and the minute hand of a clock coincide?

Accepted Answer

Introduction / Context: This problem asks for the exact time between 2:00 and 3:00 when the hour and minute hands overlap. Questions about the coincidence of clock hands are a standard topic in quantitative aptitude, because they require using a general formula for the relative motion of the hands. Understanding how often and when the hands coincide within each hour builds a deeper intuition about continuous movement on the clock dial rather than only discrete positions at exact hours. Given Data / Assumptions: - We are looking between 2:00 and 3:00. - The clock has 12 equal hour divisions, corresponding to 360 degrees total. - The minute hand moves 6 degrees per minute. - The hour hand moves 0.5 degrees per minute. - We want the moment when both hands are exactly at the same position. Concept / Approach: Let m be the number of minutes after 2:00. We compute the angular position of the hour hand and the minute hand with respect to the 12 o clock position, then set them equal because coincidence means the hands are at the same angle. This gives a linear equation in m. Solving for m yields the exact fractional number of minutes after 2:00 at which the coincidence occurs. This value is usually written as a mixed fraction like 10 10/11 minutes to match standard exam conventions. Step-by-Step Solution: Step 1: Let m be the minutes past 2:00. Step 2: At 2:00, the hour hand is at 2 * 30 = 60 degrees from 12. Step 3: In m minutes, the hour hand moves an extra 0.5 * m degrees, so its angle is 60 + 0.5 * m degrees. Step 4: The minute hand at m minutes is at 6 * m degrees from 12. Step 5: For coincidence, set the two angles equal: 60 + 0.5 * m = 6 * m. Step 6: Rearrange to get 60 = 6 * m - 0.5 * m = 5.5 * m. Step 7: Therefore m = 60 / 5.5 = 120 / 11 minutes. Step 8: 120 / 11 minutes is equal to 10 10/11 minutes, so the hands coincide at 10 10/11 minutes past 2:00. Verification / Alternative check: We can check reasonableness by recalling a general result: the hands coincide every 65 5/11 minutes and there are 11 coincidences in 12 hours. The approximate time between coincidences is a little more than one hour. After the 2:00 coincidence is reached, the next overlap will occur slightly after 3:00 plus an additional fraction of an hour. The value 10 10/11 minutes past 2:00 fits the pattern because it is the first coincidence after 1:00 and before 3:00, aligning with the known distribution of overlaps. Why Other Options Are Wrong: Values like 9 10/11 or 11 10/11 minutes past 2:00 do not satisfy the equation that equates the hour and minute hand angles. Times past 3:00 do not lie between 2:00 and 3:00, so they are automatically not valid for this question. Only 10 10/11 minutes past 2:00 arises from the correct solution of the equation 60 + 0.5 * m = 6 * m, so the other listed times are incorrect. Common Pitfalls: A frequent mistake is to assume that the overlap happens at exactly half past the hour or at some simple round minute mark. Another pitfall is forgetting that the hour hand continues to move during the minutes after 2:00, leading to equations that treat the hour hand as fixed. Always use the correct speeds of both hands, set the positions equal, and explicitly solve the resulting equation to find the exact fractional minute value. Final Answer: Between 2:00 and 3:00, the clock hands coincide at 10 10/11 minutes past 2:00.

Question 12

What is the reflex angle between the hour hand and the minute hand of a clock at 11:35?

Accepted Answer

Introduction / Context: This question focuses on finding the reflex angle between the hour and minute hands of a clock at 11:35. Unlike many clock angle problems that ask for the smaller interior angle, this one specifically asks for the reflex angle, which is the larger angle obtained by going the long way around the circle. Understanding the difference between the smaller angle and the reflex angle is important for accurately answering such problems in competitive exams and logical reasoning tests. Given Data / Assumptions: - The time is 11:35. - One full circle on the clock face is 360 degrees. - The minute hand moves 6 degrees per minute. - The hour hand moves 0.5 degrees per minute. - The reflex angle is defined as the larger of the two possible angles between the hands. Concept / Approach: To find the angle between the hands, we first compute the positions of the hour hand and the minute hand from the 12 o clock position. The minute hand position is straightforward, while the hour hand position accounts for both the hour and the extra minutes. The smaller angle is the absolute difference between these two positions. The reflex angle is then obtained by subtracting this smaller angle from 360 degrees. Our answer must match one of the given choices and be clearly labeled as the reflex angle. Step-by-Step Solution: Step 1: At 11:35, the minute hand is at 35 minutes. Step 2: Angle of the minute hand from 12 is 35 * 6 = 210 degrees. Step 3: At 11:00, the hour hand is at 11 * 30 = 330 degrees. Step 4: In 35 minutes, the hour hand moves an additional 35 * 0.5 = 17.5 degrees. Step 5: Therefore, the hour hand angle from 12 at 11:35 is 330 + 17.5 = 347.5 degrees. Step 6: The smaller angle between the hands is |347.5 - 210| = 137.5 degrees. Step 7: The reflex angle is the larger angle, which is 360 - 137.5 = 222.5 degrees. Verification / Alternative check: We can check for consistency by visualizing the clock. At 11:35, the minute hand is a little past the 7 mark on the dial, and the hour hand is near but just short of 12. The angle between them if we go the short way is clearly less than 180 degrees, making 137.5 degrees a plausible smaller angle. Subtracting this from 360 degrees gives 222.5 degrees, a reflex angle larger than 180 degrees. This matches one of the options and correctly represents the angle going around the longer side of the dial. Why Other Options Are Wrong: 137.5 degrees is the smaller angle, not the reflex angle requested by the problem. Values like 192.5 degrees and 330 degrees do not match either the direct difference or 360 minus the difference obtained from the correct hand positions. A value of exactly 180 degrees would mean the hands are in a straight line opposite each other, which does not occur at 11:35. Therefore, only 222.5 degrees is consistent with the definition of the reflex angle at this time. Common Pitfalls: One common error is to compute only the smaller angle and then select it without noticing that the question specifies reflex angle. Another pitfall is miscalculating the hour hand position by forgetting that it moves continuously during the 35 minutes after 11:00. Carefully multiplying the minutes by 0.5 degrees and then adding to the base hour position prevents these mistakes. Always double check whether the question wants the smaller angle or the reflex angle before choosing the answer. Final Answer: The reflex angle between the clock hands at 11:35 is 222.5 degrees.

Question 13

A clock is started at 11 a.m. By 600 seconds past 6 p.m, the hour hand has turned through ?

Accepted Answer

The total angle traced by the hour hand is the angle traced in 7 hours and 10 minutes. We know that the angle traced by the hour hand in one hour is 30º and in one minute is 1/2º. Therefore, (30º x 7) + (10 x 1/2º) = 215º is the angle traced by the hour hand.

Question 14

A clock loses 20 minutes every real hour. If it shows 12:00 noon now, how many minutes slow will it be when it later shows 2:00 p.m.?

Accepted Answer

Introduction / Context: This question deals with a faulty clock that does not keep correct time. Instead of advancing 60 minutes for every real hour that passes, it shows 20 minutes less each hour. The puzzle asks how many minutes the clock will be behind the correct time when it reads 2:00 p.m. after starting at 12:00 noon. Such questions test understanding of relative speed in terms of timekeeping, similar to problems about gaining or losing minutes per day on a watch. Given Data / Assumptions: - The faulty clock loses 20 minutes for each real hour that passes. - At the starting moment, both the real time and the clock reading are 12:00 noon. - We want to know the real time when the faulty clock shows 2:00 p.m. - One real hour equals 60 real minutes. - The clock is assumed to lose time at a constant rate. Concept / Approach: When a clock loses a fixed amount of time each real hour, its displayed time advances slower than real time. Here, for every 60 real minutes, the clock advances only 40 minutes, because it loses 20 minutes. This means the clock speed is 40 minutes of display per 60 minutes of real time, or two thirds of the normal rate. The relationship between displayed time and real time can be expressed as a simple proportionality, allowing us to calculate how much real time has passed when the clock shows a given display time, such as 2 hours after noon on its own face. Step-by-Step Solution: Step 1: In 60 real minutes, the clock advances only 40 display minutes. Step 2: Therefore, the ratio of real time to displayed time is 60 to 40, which simplifies to 3 to 2. Step 3: Let the displayed time interval from noon to the later reading be 2 hours, that is 120 display minutes. Step 4: Using the ratio, the corresponding real time interval is (3 / 2) * 120 = 180 real minutes. Step 5: Starting from 12:00 noon, 180 real minutes corresponds to 3 hours later, which is 3:00 p.m. Step 6: When the clock reads 2:00 p.m., the true time is 3:00 p.m. Step 7: Thus, the clock is 1 hour, or 60 minutes, slow. Verification / Alternative check: We can compute hour by hour. After 1 real hour, the correct time is 1:00 p.m. but the faulty clock shows only 12:40 p.m., so it is 20 minutes slow. After 2 real hours, the correct time is 2:00 p.m. but the faulty clock has advanced only 80 minutes, so it shows 1:20 p.m., now 40 minutes slow. After 3 real hours, the correct time is 3:00 p.m. while the faulty clock has advanced 120 display minutes from noon and now shows 2:00 p.m., which is 60 minutes behind real time. This step by step view confirms the proportional method result. Why Other Options Are Wrong: A loss of 36 or 48 minutes would correspond to smaller slowdowns than the actual 20 minutes per hour loss and do not match the proportional relationship of 3 real minutes to 2 display minutes over a two hour displayed period. A loss of 35 or 40 minutes similarly fails to align with the exact calculations. Only a loss of 60 minutes matches both the proportional reasoning and the direct hour by hour simulation. Common Pitfalls: A common mistake is to assume that after two hours on the display, only two real hours have passed, which ignores the slower running of the clock. Others subtract 20 minutes only once instead of for each real hour, leading to an underestimate of the total error. Carefully distinguishing between real time and displayed time and using either the ratio method or a consistent stepwise approach prevents these errors. Final Answer: When the faulty clock shows 2:00 p.m., it is 60 minutes slow compared to the correct time.

Question 15

How many minutes are left until 6:00 if, 50 minutes ago, the time was 4 times as many minutes past 3:00 as the number of minutes now remaining until 6:00?

Accepted Answer

Introduction / Context: This question is a variant of a standard time puzzle involving relationships between minutes past a fixed hour and minutes remaining to another fixed hour. It uses a condition relating the time fifty minutes ago to the current time left until 6:00. Solving such problems systematically builds skill in combining basic algebra with time calculations and is very useful for competitive examinations where time and work or time and distance concepts are frequently tested. Given Data / Assumptions: - Let x be the number of minutes from now until 6:00. - Fifty minutes ago, the time is 50 minutes earlier than the current time. - Six o clock is 6:00 and three o clock is 3:00. - Fifty minutes ago, the minutes past 3:00 equal four times x. - We assume a standard 12 hour clock with uniform time flow. Concept / Approach: The approach is to express all relevant times in minutes counted from a common reference, which in this case is 3:00. By defining x as the minutes remaining to 6:00, we can express the current time and the time fifty minutes ago in terms of minutes after 3:00, then use the verbal condition to write an equation. This equation is linear in x and can be solved with normal algebra techniques. The final result is then checked against the multiple choice options. Step-by-Step Solution: Step 1: Let x denote the number of minutes now remaining until 6:00. Step 2: The current time is 6:00 minus x minutes. Step 3: Fifty minutes ago, the time was 6:00 minus x minus 50 minutes. Step 4: Count minutes from 3:00. Six o clock is 180 minutes after 3:00. Step 5: The time fifty minutes ago is therefore 180 - x - 50 = 130 - x minutes after 3:00. Step 6: The condition states that 130 - x equals 4 times x, because at that earlier time the minutes past 3:00 are four times the minutes now remaining until 6:00. Step 7: Solve 130 - x = 4x to get 130 = 5x and x = 26 minutes. Step 8: Therefore, there are 26 minutes left until 6:00. Verification / Alternative check: With x = 26, the current time is 5:34. Fifty minutes earlier, the time was 4:44. From 3:00 to 4:44 is 1 hour 44 minutes which equals 104 minutes. Four times 26 is 104, so the condition given in the problem is perfectly satisfied. This consistency between the algebraic solution and the concrete clock times confirms that 26 minutes is the correct answer. Why Other Options Are Wrong: Testing 24 minutes would make the current time 5:36 and the time fifty minutes earlier 4:46, which is 106 minutes past 3:00, not 4 * 24 = 96. Similar mismatches occur for 22, 20, and 18 minutes. None of these choices satisfy the equality that the minutes past 3:00 fifty minutes ago are four times the minutes now remaining until 6:00. Only 26 minutes maintains the correct proportional relationship in both algebraic and numerical checks. Common Pitfalls: Learners often reverse the relationship and mistakenly set x equal to four times the earlier minutes, or they count from 6:00 instead of from 3:00, which produces incorrect equations. Another pitfall is misinterpreting the phrase fifty minutes ago and wrongly adding 50 instead of subtracting it. Writing down a clear time line and explicitly converting everything into minutes from a single base time helps to avoid these errors and makes such puzzles much more manageable. Final Answer: The number of minutes until 6:00 is 26 minutes.

Question 16

A siren sounds once every 5 seconds. How many times will it sound in three quarters (3/4) of an hour?

Accepted Answer

Introduction / Context: This puzzle is about counting events that occur at regular time intervals. The siren sounds once every 5 seconds, and we are asked how many times it will sound in three quarters of an hour. Such problems test your understanding of time conversions and whether you correctly interpret whether the first sounding happens at time zero or after the first interval. These types of questions appear frequently in basic arithmetic and reasoning tests. Given Data / Assumptions: - The siren sounds at a regular interval of every 5 seconds. - The total duration considered is three quarters of an hour, that is 3/4 hour. - One hour has 60 minutes, and each minute has 60 seconds. - We assume the siren sounds immediately at the start (time zero) and then at every 5 second interval afterward. - The sounding is treated as an instantaneous event at each interval. Concept / Approach: First, convert three quarters of an hour into seconds. Then determine how many 5 second intervals fit into this total duration by dividing the total seconds by 5. If we assume the siren sounds at time zero and at the end of each full interval, the total number of soundings is one more than the number of completed intervals. This interpretation is common in standard exam questions of this form, which is why the answer often differs by one from the simple division. Step-by-Step Solution: Step 1: Convert three quarters of an hour into minutes: (3 / 4) * 60 = 45 minutes. Step 2: Convert 45 minutes into seconds: 45 * 60 = 2700 seconds. Step 3: Compute the number of 5 second intervals in 2700 seconds: 2700 / 5 = 540 intervals. Step 4: If the siren sounds first at time zero and then at the end of each 5 second interval, the number of soundings equals the number of intervals plus 1. Step 5: Therefore, total soundings = 540 + 1 = 541. Step 6: Hence the siren will sound 541 times in three quarters of an hour. Verification / Alternative check: Consider a smaller example: if the siren sounds every 5 seconds over a period of 10 seconds, it would sound at time 0, time 5, and time 10 seconds. That is 3 soundings because there are two full intervals plus the initial sounding. Similarly, in 2700 seconds there are 540 full 5 second intervals and one sounding at time zero, so 540 plus 1 equals 541. This confirms that including the starting sounding is consistent with the typical interpretation of such questions. Why Other Options Are Wrong: The value 540 corresponds to counting only the completed intervals and omitting the initial sounding at time zero, which is not the usual interpretation of sounds occurring every 5 seconds. The value 450 would imply an interval larger than 5 seconds or a shorter overall time, and 275 is even further off. The option 600 does not align with the precise conversion of 45 minutes into 2700 seconds and dividing by 5 seconds per interval. Therefore, only 541 times matches the correct counting logic. Common Pitfalls: The most common error is to answer 540 by simply dividing 2700 by 5 and forgetting to consider the first sounding at time zero. Another pitfall is mixing minutes and seconds, for example dividing 45 by 5 instead of converting to seconds first. Carefully converting units and thinking about whether the event occurs at the start of the timing window as well as at the end of each interval will help avoid these traps. Final Answer: The siren will sound 541 times in three quarters of an hour.

Clock Puzzles Questions